Heretofore, in a hydraulic control device for an automatic transmission, in order to lubricate frictional engagement elements and the like of an automatic transmission, lubricating oil is supplied to lubricating required portions (bearings and the like) of the frictional engagement elements in addition to hydraulic oil for the automatic transmission. This lubricating oil is supplied directly to the inside of a lubricating oil passage from a regulator valve for regulating a line pressure, which becomes an original pressure for an engagement hydraulic oil pressure of the frictional engagement elements, for example.
Further, it is known a lubricating oil supplying apparatus for an automatic transmission, which includes a first lubricating oil passage for supplying lubricating oil from a torque converter hydraulic circuit to a lubricating required portion and a second lubricating oil passage for supplying lubricating oil from a hydraulic circuit other than the torque converter hydraulic circuit (for example, a line pressure circuit, a pilot pressure circuit, a throttle pressure circuit or the like) to the lubricating required portion (for example, see Japanese Patent Application Publication No. 3-199771 (hereinafter, referred to as “Patent Literature 1”)). In the lubricating oil supplying apparatus for the automatic transmission according to Patent Literature 1, in accordance with a driving status of a vehicle, lubricating oil is supplied to the lubricating required portion via the first lubricating oil passage at a low load, and lubricating oil is also supplied from the second lubricating oil passage in addition to the first lubricating oil passage at a high load in order to increase the amount of lubricating oil.
In this lubricating oil supplying apparatus for the automatic transmission disclosed in Patent Literature 1, a solenoid valve that communicates the first lubricating oil passage with the second lubricating oil passage and blocks off the first lubricating oil passage from the second lubricating oil passage is provided between the first lubricating oil passage and the second lubricating oil passage. By controlling this solenoid valve to open or close, the hydraulic oil and/or the drained hydraulic oil are supplied to the lubricating required portion from a hydraulic circuit other than the torque converter hydraulic circuit via the second lubricating oil passage when a high load is applied onto the vehicle.